Scanner, Observer, Pilot flying, Pilot not flying, and even driving. It is important to keep your eyes moving and not fixate (stare) at any single point or instrument too long (measured in secionds not minutes). Here is a short piece from another forum:

This test is a reminder of what can happen to our vision. We should keep our eyes moving in the cockpit to avoid this.

Thats me in the door, scanning, on the turn to jump run. As you can see, nobody else is looking out of the plane.

I use the horizontal strips of the door as guides for horizontal scanning for potential intersecting traffic 8,000-10,000 ft below us. I start looking on the downwind/crosswind leg (often toward KASH, which is about 6 miles NE of our field) to cover anybody outbound, then as we pull the U-turn/dog-leg onto jump run, I'm looking for the guys making their way _to_ KASH.

It works pretty well, plus all the scanning techniques I learned in the Army and in CAP (offcenter vision, anybody?). It can be pretty hard to spot a moving plane against the ground from 2 miles up, but I do it pretty frequently.

So yeah, that would make an outstanding safety briefing. Matter of fact, I think I might make that a topic for Safety Day next year.

+1 to what NIN said. When I let jumpers out of my aircraft, I am very persistent when it comes to aircraft flying over the field. Most pilots don't understand the fact that a skydiver will be ~5,000ft a minute after jumpers away. Also more crucial is when jumpers are down around pattern altitude, canopies are easy to see when looking up from the ground against a blue sky, but difficult when at the same altitude or above, even if your canopy is safety green.

+1 to the OP, on a blue sky day, without a define horizon, it is easy to have empty field myopia. As a pilot, your scan is crucial to see and avoid, even when operating IFR, see and avoid must by used whenever possible.

Lastly, if you are a member of an aircrew or a passenger, it may seem like a no brainer, but if you see another plane, TELL the pilot! its better for him/her to say I see it thanks anyway then have a possible mid-air. Its better to have 4+ eyes looking for traffic instead of just two.

"We praise our brain in many ways, but that's surely because it's our brain. But we're so easily fooled by other humans and even by nature itself, that we need the methods and tools of science to shield us from bias, blunder and even delusion. If we were honest with ourselves, then, for example, books on optical illusions would really be called brain failures." -- Dr. Neil deGrasse Tyson